Media access devices for personal computers (PCs) have proliferated in the last few years. A large number of media access devices such as modems, LAN adapters and specialized wireless communication cards currently under development are based on the PCMCIA standard (as defined in the PCMCIA I/O Card Interface Specification available from Personal Computer Memory Card International Association of 10308 East Duane Avenue, Sunnyvale, Calif. 94086, which is incorporated herein by reference). Such media access devices for laptop PCs typically use battery based power supplies. When not in use, such media access devices act as a drain on the battery. It is desirable to switch off the power supply to a media access device when the device is not being used. Switching off of the power supply is preferably done automatically to avoid cumbersome manual switches and to avoid battery drain in cases when the user forgets to switch off the device. It is also desirable that this automatic operation be done via one of the pins of the media access device in order to avoid providing a separate line just for power supply switching. Under the PCMCIA standard, although there is a POWERB pin defined as pin 4, it is allocated for power control of a DAA, telephone interface, and is not available for switching power on the media access device. Since all the pins are allocated by the PCMCIA standard, it is desirable that an energy conservation circuit in the media access device be responsive to messages sent on an asynchronous serial data line.
An energy conservation circuit inside a media access device must meet three important criteria for conserving battery power. First, the energy conservation circuit must power down the device when so instructed on an internal shutoff line. Second, in order to conserve scarce battery power, the energy conservation circuit and the rest of the device must draw absolutely no power once the media access device has been powered down. Third, the energy conservation circuit must power up by itself and turn on the power to the rest of the device when instructed to do so on the serial data transfer pin even though there is no power in the media access device. These three criteria are necessary for saving battery power in laptop PC media access devices.
The first criterion is relatively straightforward to implement in a media access device via a circuit that turns itself off on receipt of a shutdown signal. However, the second criterion is not satisfied by most present day sleep/wake-up circuits since they use some power during the sleep mode to monitor the host device. Any non-negligible power consumption during the sleep mode can add up over extended periods of time resulting in an unnecessary drain on the battery. The reason most sleep/wake-up circuits monitor the host device is the difficulty in satisfying the third criterion. It is not a trivial matter for any device to turn back on again after being powered down. When a device is completely powered off and there is absolutely no power in the device, none of the circuits in the device can respond to external messages.